How do you annotate variables, parameters and return types?

Question

Accepted Answer

You add a type after a colon. TypeScript can **infer** many types, so you annotate where it adds clarity or where inference isn't possible.

```ts
// variables — usually let inference work
let count: number = 0;     // explicit
let name = "Ann";          // inferred as string (annotation redundant)

// function parameters — always annotate (can't be inferred)
function greet(name: string, times: number): string {
  return name.repeat(times);
}

// arrays and objects
const ids: number[] = [1, 2, 3];
const user: { name: string; age: number } = { name: "Ann", age: 30 };
```

## Return types: infer or annotate?

```ts
function add(a: number, b: number) { return a + b; } // inferred return: number
function load(): Promise<User> { ... }                // explicit — documents the contract
```

Return types are inferred, but annotating them on exported/public functions is good practice: it documents intent and catches mistakes *inside* the function (if you accidentally return the wrong thing, the error points at the function, not its callers).

## Special cases

```ts
function log(msg: string): void {}     // returns nothing
function fail(): never { throw new Error(); } // never returns normally
const fn: (x: number) => string = ...; // function type annotation
```

## Why it matters

The rule of thumb: **annotate function signatures (parameters and public return types) explicitly, let local variables be inferred.** This keeps code concise while making the contracts at function boundaries clear and checked.